Photochromic dyes are known to undergo reversible color changes on exposure to ultraviolet light. U.S. Pat. Nos. 3,341,330, 3,355,293 and 3,346,385 teach how to form persistent images from thin layers of dyes and how to further treat them with NO.sub.2. Similar technology is known in photochromic photography (color photography).
Chemichromic dyes are probably best known by analytical chemists who use pH sensitive dyes to indicate pH and dyes sensitive to specific elements or oxidation states thereof to make calorimetric test methods for determining quantitatively compounds such as iron, amines and lead.
Piezochromic dyes are Compounds which upon exposure to different compressive or tensile forces alter or shift the frequencies of light which they absorb.